Telephone headset amplifiers provide an interface between the telephone and the headset worn by the telephone operator. The headset amplifier receives the audio signal from the telephone, limits the maximum amplitude of the audio signal to improve operator safety, and provides a power output to drive the earphone part of the telephone headset. The headset provides power for the headset microphone, and takes the audio signal from the microphone, switches the gain of the audio signal from the microphone, and drives the gain-switched signal from the microphone into the telephone line through a 4 to 2 wire convertor in the telephone. Sidetone is generated in the 4 to 2 wire convertor enabling the operator to hear through the earphone what he/she is saying into the microphone.
Headset amplifiers can be powered from the a.c. line by an a.c. to D.C. power supply, or they can draw power from the telephone line. Modern headset amplifiers are battery powered, usually by two "AA"-size alkaline cells. Batteries provide a convenient power source for a telephone headset amplifier, but, since they have finite life, powering a telephone headset from batteries can be relatively expensive, and can also present reliability problems. It has proved difficult to train telephone operators to switch the headset amplifier off when the operator leaves the workstation. Failure to switch the amplifier off increases cost, because the batteries continue to provide current while the amplifier is not being used. Failure to switch the amplifier off also reduces reliability, because the batteries of an amplifier left on overnight or over the weekend are less serviceable, which forces the replacement of batteries often enough to become an annoyance.
Gain switching is applied to the microphone output to reduce pickup of extraneous sounds when the operator is not speaking. Additionally, it is desirable to be able to mute the microphone output briefly at times such as when the operator has to sneeze or cough. Known muting circuits tend to cause an annoying click or pop on the transmit line when the operator operates the mute switch.
Between calls on many telephone systems on which headsets are used continuously there is idle channel noise. The operator must listen to this noise while nobody is speaking on the line. The operator cannot remove or disconnect his/her headset to avoid listening to this noise, because the operator must be ready at all times to deal with incoming calls. Such noise is undesirable since it causes operator fatigue and reduces operator efficiency.